There are many different instances where photoimagable compositions are utilized in various industrial processes, such as circuit board manufacturing. For example, photoimagable compositions are utilized as solder masks or as a dielectric, or as both by applying the photoimagable composition to the underlying substrate such as conventional epoxy glass laminate. Thereafter photolithographic techniques are employed to reveal various underlying structures on the board while masking others so that solder may be applied to the exposed structures. During the solder applying process the solder will adhere to the exposed underlying components and be prevented from adhering where the remaining material operates as a solder mask.
In a quest for miniaturization, attempts have been made to condense more lines and structures onto the circuit board. The lines and structures must therefore be finer and closer together. The proximity of lines and structures creates a variety of problems; for example, in soldering chips onto pads, care must be taken to protect and isolate the conductive structures so as to prevent solder bridges from forming between structures. However, as the area to be protected by a solder mask decreases in size, conventional solder masks often demonstrate an inability to adhere to conventional epoxy glass substrates, such as "FR4 boards". If the mask detaches, thereby exposing underlying area, solder is likely to spill onto the unprotected area and form an unwanted bridge between the structure being soldered and the exposed structure thereby creating a short circuit. In an effort to improve the adherence between the solder mask and the structures to be protected, those in the art have sought to improve the solder mask composition.
However, adherence to the substrate is not the only consideration in formulating a solder mask. It is necessary that the solder mask material be formulated such that it can be applied by the appropriate methods; for example curtain coating and screen coatings require certain rheological properties for effective coating. Further, the solder mask must provide efficient transmission of the light or other exposing radiation so as to photolyze the photoinitiator through whatever thickness of material is applied. Also, if the material is to be used as a solder mask, the solder mask must possess appropriate physical and chemical properties to withstand the application of the solder material without significant deterioration, degradation or loss of adhesion to the underlying substrate and maintain its coverage over the portions of the board wherein solder is to be masked.
Good photopatternable formulations which permit high resolution fine line photopatterning have been disclosed in the parent application Ser. No. 07/318,536, which issued as U.S. Pat. No. 5,026,624. However, the high resolution fine line photopatterning of such formulations on the conventional "FR4" circuit boards, is limited.
It is desirable to have a liquid photoimagable coating which may be used as a solder mask or a dielectric, or both, containing epoxy resin cured with photosensitive cationic initiators which demonstrate good image resolution on conventional epoxy-glass substrates.